High-bypass-ratio turbofan engines are used to power modern large commercial aircraft because of their overall efficiency, high thrust at low flight speeds, low jet velocity and low fuel consumption. A fan containment shroud is the largest structural component in these engines. The containment shroud is intended to contain a fan blade in the rare event of a blade loss during engine operation (the centrifugal force acting on a broken blade can cause the blade to puncture the engine nacelle). In addition, the containment shroud should retain its structural integrity after a blade-out incident to limit secondary damage caused by impact debris and to constrain out-of-balance motion of the engine's rotor after a blade or blade fragment is lost.
Two approaches are currently used to contain fan blades in modern commercial engines. The first approach (hardwall shroud design) uses an impact resistant metal alloy for a fan case with a wall thickness sufficient to prevent perforation by a fan blade and circumferential ribs for stiffness. In the hardwall shroud design, the wall thickness is much greater than the thickness required for structural loads during normal engine operation. The extra weight, which is needed only in the rare event of a blade-out, is therefore a parasitic weight that reduces overall energy efficiency during normal engine operation. The second approach (softwall shroud design) uses a thin metal alloy case that provides structural capabilities, while containment capability is provided by a fiber or fabric wrap (Kevlar®) around the case. During a blade-out event, the blade passes through the inner metal case and is captured in the outer fabric layers. Case stiffness may be provided by an isogrid rib pattern in the metal case or by using a honeycomb structure on the outside of the case. In the softwall design, the fabric wrap is a parasitic weight.
In U.S. Pat. No. 6,003,424, there is described an armor system that is said to be useful as a containment shroud for a fan-blade engine. The armor system comprises a first pliable, cut resistant fibrous layer and a second pliable fibrous layer substantially coextensive with and surrounding the first layer. The first layer is intended to engage any projectile thrown by the engine to slow its velocity, and the second layer is intended to dissipate the incoming energy and thereby resist complete penetration of the second layer by the projectile. The first layer may comprise a plurality of networks selected from the group consisting of an uncoated nonwoven network of randomly oriented fibers and an uncoated knitted, preferably tightly, network of fibers. The second layer may comprise a plurality of networks selected from the group consisting of a loosely woven network of fibers, an open knitted network of fibers, a braided network of fibers, and a nonwoven network of oriented fibers.
The '424 patent indicates a wide variety of metallic, semi-metallic, inorganic and/or organic fibers can be used, while stating it is crucial that a sufficient weight percent of cut resistant fibers or combination of fibers with high tensile properties be used to achieve the indicated properties of the layers of the armor systems. Fibers having the high tensile properties desired are those having a tenacity equal to or greater than about 10 g/d, a tensile modulus equal to or greater than about 200 g/d and an energy-to-break equal to or greater than about 8 Joules/gram (J/g), with the fibers of choice having a tenacity equal to or greater than about 35 g/d, the tensile modulus equal to or greater than about 1500 g/d and the energy-to-break equal to or greater than about 50 J/g. According to the '424 patent, these highest values for tenacity, tensile modulus and energy-to-break are generally obtainable only by employing solution grown or gel filament processes, such as are used to produce high strength polyethylene fiber known as Spectra.RTM, a product of Allied-Signal, Inc.
While the fibers of the first layer are uncoated, the fibers of the second layer are coated with a matrix material. According to the '424 patent, the proportion of matrix to filament in the network preferably is about 10 to 30%. The network preferably is comprised of a plurality of sheet-like arrays of untwisted fibers with the fibers aligned substantially parallel to one another along a common fiber direction within each array. The arrays are preferably individually impregnated with a matrix binder and stacked. According to the '424 patent, it is important that the arrays not be consolidated between networks with the matrix binder since this will stiffen the layer too much.
Heretofore it also has been proposed to use a ceramic layer on the impact surface of a composite wall of a containment shroud. The ceramic layer serves to fragment the projectile and spread the impact load over a larger area of the composite backing. According to U.S. Pat. No. 6,113,347, an annular fan containment shroud may have an interior surface with an abrasive surface texture that is capable of dulling sharp corners and edges of impacting fan blades so as to reduce the ability of the fan blade to pierce the containment shroud. The textured surface may be produced by protuberances in the form of sawtooth steps, sharp spikes or pieces of a hard material impregnated in the textured surface.
The present invention was developed to overcome the deficiencies of prior art jet engine fan blade containment shrouds and provide superior stopping power for the same or less areal weight.